The main two differences between any of the telecom
standards are the frequency bands of operation and
the modulation techniques. Some use frequency modulation,
others use phase modulation, but all are some form
of digital modulation. Just as there are disparate
groups of auto enthusiasts that fervently believe
their choice of car is the best, groups exist in
the mobile communications cadres that think theirs
is the best. Some systems are better than other
form the standpoint of bandwidth efficiency, but
that often comes with a degradation in call quality
- particularly in weak signal conditions.
Take for example 16-QAM, where the receiver
must correctly discern 16 unique phase angle/amplitude
combinations to properly decode a data package.
With a strong signal and a slow-moving platform
(a walking talker), a phone can usually get near
100% of the data packages without the need for error
correction or maybe have to request that the packet
be re-sent from the tower. Same goes in the other
direction from talker to tower.
Wne a user
is moving quickly in a low power enviroment with
lots of signal interferes around (electronics and
physical), the system often breaks down. That is
because of ambiguity in phase, amplitude. Take a
simple BPSK signal in the same environment that
only needs to discern between two separate states
and it has a much better chance of surviving. Problem
is, though, is that BPSK takes 8x the BW of a 16-PSK
You can look up the technical difference
between the various mobil phone systems, but in
a nutshell this is the difference.